CAT661

9

Doc. No. 5003, Rev. J

VRIPPLE (mV)

IOUT (mA)

FOSC (kHz)

C2 (

µF)

C2 ESR (

Ω)

45

100

25

150

0.2

25

100

135

150

0.2

CAPACITOR SELECTION

Low ESR capacitors are necessary to minimize voltage

losses, especially at high load currents. The exact

values of C1 and C2 are not critical but low ESR

capacitors are necessary.

The ESR of capacitor C1, the pump capacitor, can have

a pronounced effect on the output. C1 currents are

approximately twice the output current and losses occur

on both the charge and discharge cycle. The ESR

effects are thus multiplied by four. A 0.5

Ω ESR for C1 will

have the same effect as a 2

Ω increase in CAT661 output

impedance.

Output voltage ripple is determined by the value of C2

and the load current. C2 is charged and discharged at a

current roughly equal to the load current. The internal

switching frequency is one-half the oscillator frequency.

VRIPPLE = IOUT/(FOSC x C2) + IOUT x ESRC2

For example, with a 25kHz oscillator frequency (12.5kHz

switching frequency), a 150

µF C2 capacitor with an ESR

of 0.2

Ω and a 100mA load peak-to-peak the ripple

voltage is 45mV.

VRIPPLE vs. FOSC

Figure 6. Oscillator

Figure 7. External Clocking

BOOST/FC

(1)

LV

(6)

OSC

(7)

~18pF

I

7.0 I

7.0 I

I

V+

+

-V+

C2

1

2

3

4

8

7

6

5

REQUIRED FOR TTL LOGIC

CAT661

V+

100k

OSC INPUT

NC

+

C1

BOOST/FC

CAP+

GND

CAP-

V+

OSC

LV

OUT